Scattered-light smoke detectors measure the amount of light scattered by an aerosol in order to infer the smoke density, and hence indicate a fire. In a typical such detector, a photo detector and a light source are mounted within a chamber into which the aerosol is admitted. A partly collimated beam of light is passed through the aerosol but not allowed to shine directly onto the photo detector. When the aerosol contains sufficient concentrations of smoke particles, the light emitted by the source is scattered by the smoke, a small portion of which will be incident on the photo detector. The output response of the photo detector is used to infer the smoke density. The amount of light incident on the photo detector, when smoke density reaches levels typical of a fire, is around 1/100,000 to 1/10,000 of the quantity of light emitted by the source.
The chamber's principal function is to prevent light which has not been scattered by smoke from reaching the photo detector, i.e. it must be dark. One contribution to stray light is reflections from the chamber walls. Although the walls may be black, there is still a substantial amount of reflected light compared with the levels scattered from the smoke. Reflections from the chamber walls increase with time as dust enters the chamber and coats the walls.
Another contribution to stray light is from external light. The detector must necessarily permit external air and air/smoke aerosols to reach the chamber. However, passages which allow air to flow into the chamber may also permit light to enter, for example via reflections.
In order to ensure proper function of the sensor therefore, the chamber must: allow air and air/smoke aerosols to reach the sensor; prevent significant amounts of light being reflected from the source onto the photo detector when there is no smoke present; and continue to prevent these reflections even after long term use in dusty environments.
One way of reducing the effect of reflected light is to use a light source with a narrow beam having small divergence. Granted GB patent GB2404731 discloses a surface mount device (SMD) comprising an LED chip device which produces a narrow light beam having a low divergence within a small envelope.